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Physics Assertion (A) & Reason (B) MCQ

Nuclei · Uttarakhand Board (UBSE) STD 12 Science (Uttarakhand - English Medium). 14 questions.

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Assertion (A) & Reason (B) MCQ

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14 questions · 3 auto-graded MCQ + 11 self-marked written.

Question 11 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): $_7^{14}\text{N}$ is stable.
Reason (R): Nuclei having an odd number of protons and an odd number of neutrons are generally less stable than the one having even number of protons and even number of neutrons.
Answer
  1. Both A and R are true, but R is NOT the correct explanation of A.
Explanation:

Nitrogen is a stable element. Those elements which have an odd number of protons and an odd number of neutrons are generally unstable. $_7^{14}\text{N}$ has 7 protons and 7 neutrons i.e., odd number of both protons and neutrons, but it is an exception. It is a stable element.
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Question 21 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Density of all the nuclei is same.
Reason (R): Radius of nucleus is directly proportional to the cube root of mass number.
Answer
  1. Both A and R are true, and R is the correct explanation of A.
Explanation:

Experimentally, it is found that the average radius of a nucleus is given by $\text{R}=\text{R}_0\text{A}\frac{1}{3}$ where $\text{R}_0=1.1\times10_{15}$and A = mass number

The volume of a nucleus is $\text{V}=\frac{4}{3}\pi\text{R}^3=\frac{4}{3}\pi\text{R}_0^3\text{A}$ Now as the masses of a proton and a neutron are roughly equal, say m, the mass of a nucleus is also roughly proportional to the mass number A, M = mA Hence density within a nucleus, $\rho=\frac{\text{M}}{\text{V}}-=\frac{\text{mA}}{\frac{4}{3}\pi\text{R}_0^3\text{A}}$

$=\frac{\text{m}}{\frac{4}{3}\pi\text{R}_0^3}$
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Question 31 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Energy is released in a nuclear reaction.
Reason (R): In any nuclear reaction the reactants and resultant products obey the law of conservation of charge and mass only.
Answer
  1. A is true, but R is false.
Explanation:

In both fission and fusion, large amounts of energy is released. The reason is correct. Charge, mass, momentum and energy, all are conserved
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Question 41 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Cadmium rods, used in a nuclear reactor, control the rate of fission. Reason (R): Cadmium rods, used in a nuclear reactor, control the rate of fission.
Answer
  1. A is true, but R is false.
Explanation:

Cadmium rods are used in a nuclear reactor to control the rate of fission. The cadmium rods do not slow down or speed up the neutrons produced in a fission reaction of $^{235}U$. Instead, they absorb the neutrons, thereby regulating the power level of the reactor.
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Question 51 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): A fission reaction can be more easily controlled than a fission reaction.
Reason (R): The percentage of mass converted to energy in a fission reaction is 0.1% whereas in a fission reaction it is 0.4%
Answer
  1. Both A and R are true, but R is NOT the correct explanation of A.
Explanation:

Percentage of mass converted to energy in a fission reaction is 0.1% whereas in a fusion reaction it is 0.4%. Consequently, the amount of energy released is more in a fusion than in a fission reaction. II is not easy Jo control a fusion reaction.
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MCQ 61 Mark
For question, statements are given-one labelled Assertion $(A)$ and the other labelled Reason $(R)$. Select the correct answer to these questions from the codes $(a) (b) (c)$ and $(d)$ as given below.
Assertion $(A):$ Nuclear density is extremely higher than atomic density.
Reason $(R):$ Most of the mass of the atom is concentrated in the nucleus.
  • ✓
    Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
  • B
    Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$
  • C
    $A$ is true but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: A.
Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
According to the planetary model of the atom, the mass of the atom is concentrate at the centre, the nucleus.
The electrons orbit around the nucleus in circular paths of different radii.
The radius of the outermost orbit gives the size of atom.
The size of the atom is $\sim 10^{-10} \mathrm{~m}$ as compared to the size of the nucleus, $10^{-15} \mathrm{~m}$.
Electrons being extremely light $\left(m_e=5.49 \times 10^{-4} \mathrm{u}=9.11 \times 10^{-31} \mathrm{~kg}\right)$ as compared to protons and neutrons have a negligible contribution to the weight of the atom.
They mainly increase the volume of the atom.
Therefore, the nuclear density is much higher that the atomic density $\left(\frac{10^3 \mathrm{~kg}}{\mathrm{~m}^3}\right)$
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Question 71 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Isotopes of an element can be separated by using a mass spectrometer.
Reason (R): Separation of isotopes is possible because of the difference in electron numbers of isotopes
Answer
  1. A is true, but R is false.
Explanation:

Isotopes have same number of electrons and protons, but different neutron number. That's why the mass number of isotopes are different and can be separated by using a mass spectrometer.
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Question 81 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): The nuclear force becomes weak if the nucleus contains too many protons compared to neutrons.
Reason (R): The electrostatic forces weaken the nuclear force.
Answer
  1. A is true, but R is false.
Explanation:

Nuclear forces are strongest when the number of protons equals the number of neutrons. An excess of protons compared to neutrons weakens the nuclear force. Also, too many neutrons compared to protons inside the nucleus weaken the nuclear forces. The electrostatic force which is a hundred times less than the nuclear force is not the cause.
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MCQ 91 Mark
For question, statements are given-one labelled Assertion $(A)$ and the other labelled Reason $(R).$ Select the correct answer to these questions from the codes $(a) (b) (c)$ and $(d)$ as given below.
Assertion $(A):$ Two protons can attract each other.
Reason $(R):$ The distance between the protons within the nucleus is about $10^{-15}m$
  • ✓
    Both $A$ and $R$ are true, and $R$ is the correct explanation of $A$.
  • B
    Both $A$ and $R$ are true, but $R$ is not the correct explanation of $A$.
  • C
    $A$ is true, but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: A.
Both $A$ and $R$ are true, and $R$ is the correct explanation of $A$.
Due to electrostatic forces between two protons $($like charges$)$ there is a force of repulsion. However, when the distance between them is $~ 10-15m$ they come under the influence of the short range, strong nuclear forces. $($The range of the nuclear forces is $~ 10^{-15}m).$ These forces are attractive forces and charges independent. The net force on the protons is attractive, as nuclear forces are much stronger than electrostatic forces. The protons attract each other.
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Question 101 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): For the fission of heavy nuclei, neutrons are more effective than protons.
Reason (R): Neutrons are heavier than protons.
Answer
  1. Both A and R are true, but R is NOT the correct explanation of A.
Explanation:

A neutron is slightly heavier than a proton. As neutrons are charge less particles, they penetrate matter more than protons. Therefore, they are more effective than protons in fission reactions.
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Question 111 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Thermonuclear fusion reactions may become the source of unlimited power for the mankind.
Reason (R): A single fusion event involving isotopes of hydrogen produces more energy than energy from nuclear fission of a single uranium.
Answer
  1. A is true, but R is false.
Explanation:

When fusion is achieved by raising the temperature of the system so that particles have enough kinetic energy to overcome the coulomb repulsive behaviour, it is called thermonuclear fusion. It is clean source of energy, but energy released in one fusion is much less than a single uranium fission.
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MCQ 121 Mark
For question, statements are given$-$one labelled Assertion $(A)$ and the other labelled Reason $(R).$ Select the correct answer to these questions from the codes $(a) (b) (c)$ and $(d)$ as given below.
Assertion $(A):$ The ratio for time taken for light emission from an atom to that for release of nuclear energy in fission is $1 : 100.$
Reason $(R):$ Time taken for the light emission from an atom is of the order of $10^{-8}$ s.
  • ✓
    Both $A$ and $R$ are true, and $R$ is the correct explanation of $A.$
  • B
    Both $A$ and $R$ are true, but $R$ is not the correct explanation of $A.$
  • C
    $A$ is true, but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: A.
Both $A$ and $R$ are true, and $R$ is the correct explanation of $A.$
Time taken for the light emission from an atom $\approx10^{-8}\text{s}$
Time taken for release of energy in fission $\approx10^{-6}\text{s}$
$\text{Required ratio}=\frac{10^{-8}}{10^{-6}}=\frac{1}{100}=1 :100$
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Question 131 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): Rydberg's constant varies with the mass number of a given element.
Reason (R): The reduced mass of the electron depends on the mass of the nucleus only.
Answer
  1. A is false and R is also false.
Explanation:

The Rydberg constant is given by $\text{R}=\frac{13.6\text{eV}}{\text{hc}}$

Which is independent of the mass number of an element. Reduced mass of the system depends on the masses of both the electron as well as nucleus.
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Question 141 Mark
For question, statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a) (b) (c) and (d) as given below.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is NOT the correct explanation of A.
  3. A is true, but R is false.
  4. A is false and R is also false.
Assertion (A): There is a chain reaction when uranium is bombarded with slow neutrons. Reason (R): When uranium is bombarded with slow neutrons, more neutrons are produced.
Answer
  1. Both A and R are true, and R is the correct explanation of A.
Explanation:

When uranium is bombarded by slow neutrons the reaction is represented as $_{92}^{235}\text{U}+\ _0^1\text{n}\rightarrow\ _ {56}^{141}\text{Ba}+\ _{36}^{92}\text{kr}+\ 3_0^1\text{n}+\text{Energy}$

As more neutrons are produced, the reason is correct. These additional neutrons strike other uranium nuclei to produce even more neutrons. Thus a chain reaction is established.
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